1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a particle size distribution of suspended particulate matter existing in the atmospheric air. More particularly, the present invention relates to a method and an apparatus for measuring suspended particulate matter capable of measuring a particle size distribution of the suspended particulate matter in a wide range of particle size at high resolution.
2. Description of the Related Art
Suspended dust in the atmospheric air, the particle size of which is not more than 10 xcexcm, is referred to as suspended particulate matter (SPM). Although this suspended particulate matter contains scattered mud, most of the suspended particulate matter is black smoke, which has been exhausted from Diesel engine cars, unburnt fuel and sulfur compounds. In the Kanto District, about 35% of the suspended particulate matter is exhausted from Diesel engine cars. It is said that the suspended particulate matter is harmful to human""s health. Especially, particulate matter, which is contained in exhaust gas exhausted from Diesel engine cars, is referred to as DEP (Diesel Exhaust Particles). Particulate matter of small particle size, the size of which is not more than 2.5 xcexcm, is referred to as fine particulate matter (PM 2.5), which has been actively investigated in Europe and America. It is said that most of this PM 2.5 is exhausted from Diesel engine cars.
As an apparatus for measuring a particle size distribution of the above suspended particulate matter (SPM) or fine particulate matter (PM 2.5) in the atmospheric air, a measuring apparatus based on the cascade impacter system has been conventionally put into practical use. In this measuring apparatus based on the cascade impacter system, fluid including particulates is made to collide with a collecting plate so as to suddenly change a direction of the current of fluid, so that the particulates can be separated from the fluid. In this measuring apparatus, impacters, the particle sizes at the collecting efficiency of 50% of which are different each other, are connected with each other in series so that they can be formed into a multiple stage. Then, the particle size at the collecting efficiency of 50% of each stage is used as a representative particle size, and a particle size distribution in the fluid is found from the result of measurement of collection of particulates having the representative particle size at each stage.
In this connection, in the measuring apparatus according to the cascade impacter system used for the measurement of SPM and PM 2.5, the following problems may be encountered. An upper limit of measuring the particle size is set at about 10 xcexcm in the principle, and further resolution of the particle size is determined by the number of the collecting plates. Therefore, it is impossible to measure a particle size distribution at high resolution.
It is an object of the present invention to provide a method and an apparatus for measuring suspended particulate matter capable of measuring a distribution of particle size of suspended particulate matter (SPM) and fine particulate matter (PM 2.5) in the atmospheric air in a wide range of particle size including the particle size not less than 10 xcexcm at high resolution.
In order to accomplish the above object, the present invention provides a method for measuring suspended particulate matter in the atmospheric air, the method comprising: sucking suspended particulate matter in the atmospheric air into a container; electrically charging the suspended particulate matter in the atmospheric air in the container; electrically collecting the charged suspended particulate matter; irradiating the collected suspended particulate matter with laser beams under the condition that the collected suspended particulate matter is dispersed; measuring a spatial intensity distribution of diffracted and scattered light obtained by irradiating the suspended particulate matter with laser beams; and calculating a particle size distribution of the suspended particulate matter from the result of the measurement.
The method for measuring suspended particulate matter in the atmospheric air according to the present invention, preferably, further comprises: dispersing the collected suspended particulate matter in medium fluid, wherein the suspended particulate matter dispersed in the medium fluid is irradiated with the laser beams.
Alternatively, the method for measuring suspended particulate matter in the atmospheric air according to the present invention, preferably, further comprises: causing the suspended particulate matter to adhere onto a surface of a transparent plate in the container, wherein the transparent plate, onto which the suspended particulate matter adheres, is irradiated with laser beams. Any of the above methods can be adopted.
In order to accomplish the above object, the present invention also provides an apparatus for measuring suspended particulate matter in the atmospheric air, the comprising: a container; a pump for sucking the atmospheric air into the container; a discharging electrode, which is arranged in the container, for electrically charging the suspended particulate matter in the container by generating single-pole ions; a dust collecting electrode, which has an electric potential difference with respect to the discharging electrode, for collecting the suspended particulate matter, which has been electrically charged in the container by the discharging electrode; a dispersing mechanism for dispersing the suspended particulate matter collected by the dust collecting electrode; an irradiating optical system for irradiating the dispersed suspended particulate matter with laser beams; a measuring optical system for measuring a spatial intensity distribution of light which has been diffracted and scattered by the suspended particulate matter when the suspended particulate matter is irradiated with laser beams; and a calculation section for calculating a particle size distribution of the collected suspended particulate matter from the result of measurement.
According to the apparatus for measuring suspended particulate matter according to the present invention, it is preferable that the dispersing mechanism has a dispersing tank including medium fluid accommodated therein. Alternatively, it is also preferable that the dispersing mechanism has a transparent plate, onto a surface of which the suspended particulate matter adheres.
The pump, preferably, includes a pneumatic machine capable of sucking and feeding atmospheric air forcibly. Specifically, the pump includes a compressor or blower.
As medium solution in which the collected suspended particulate matter is dispersed, it is possible to use clean water such as distilled water, organic solvent or organic solvent to which a dispersing agent such as a surface active agent is added.
According to the present invention, the measurement of a particle size distribution based on the method of laser beam diffraction and scattering, which is capable of measuring the particle size distribution at high resolution in a wide particle size range, is applied to the measurement of a particle size distribution of suspended particulate matter in the atmospheric air. Further, in order to obtain a sufficiently high intensity of diffracted and scattered light in the irradiation of laser beams, suspended particulate matter in the atmospheric air is not directly irradiated with laser beams but suspended particulate matter is irradiated with laser beams under the condition that the suspended particulate matter is effectively collected and diffused at a sufficiently high concentration so as to accomplish the object of the present invention.
In the case of a particle size analyzer based on the laser diffraction method, in general, a spatial intensity distribution of diffracted and scattered light obtained by the irradiation of laser beams to a group of particulates in a scattered condition is measured. By utilizing that the light intensity distribution agrees with Mie""s Scattering Theory or Fraunhofer""s Diffraction Theory, a particle size distribution of the group of particulates to be measured is found by the calculation based on Mie""s Scattering Theory or Fraunhofer""s Diffraction Theory according to the result of the measurement of the spatial intensity distribution of diffracted and scattered light. According to, this particle size analyzer based on the laser diffraction method, when the concentration of the group of particulates to be measured in the medium, in which the group of particulates to be measured is scattered, is kept in an appropriate range, it possible to find a particle size distribution at high resolution in a wide particle size range.
However, in the case of suspended particulate matter in the atmospheric air, even when diffracted and scattered light is measured by directly irradiating laser beams to the suspended particulate matter in the atmospheric air, it is impossible to obtain a sufficiently high intensity of diffracted and scattered light for finding the particle size distribution of the suspended particulate matter because the concentration of suspended particulate matter in the atmospheric air is too low.
Therefore, according to the present invention, the spatial intensity distribution of diffracted and scattered light obtained by the irradiation of laser beams to suspended particulate matter is measured as follows. The atmospheric air is sucked into a container, and suspended particulate matter contained in the atmospheric air in the container is electrically collected. That is, the discharging electrode generates single-pole ions, and suspended particulate matter is electrically charged by the single-pole ions generated from the discharging electrode. Then, a dust collecting electrode, to which a predetermined electric potential difference with respect to the discharging electrode is given, is arranged, and the electrically charged suspended particulate matter is collected by the dust collecting electrode. The thus collected suspended particulate matter is dispersed into a concentration range appropriate for the particle size distribution analysis based on the laser diffraction method. The thus dispersed suspended particulate matter is irradiated with laser beams, and the spatial intensity distribution of diffracted and scattered light is measured. Due to the foregoing, it is possible to find a particle size distribution of suspended particulate matter at high resolution in a wide particle size range which is identical with that of the common particle size analysis based on the laser diffraction method. That is, it is possible to find a particle size distribution of suspended particulate matter at high resolution in a wide particle size range from sub-micron to 10 xcexcm or greater.
Preferably, the collected suspended particulate matter is dispersed in medium fluid and the suspended particulate matter is irradiated with laser beams in a dispersed state so as to measure diffracted and scattered light. Alternatively, the suspended particulate matter is being collected, while adhering onto a surface of a transparent plate so that the suspended particulate matter is dispersed. Then, the suspended particulate matter is irradiated with laser beams in a dispersed state so as to measure diffracted and scattered light. By any of these methods, diffracted and scattered light, which has been diffracted and scattered by the collected suspended particulate matter, can be measured with high accuracy.
According to the present invention, since the atmospheric air is sucked into a container by a pump and suspended particulate matter contained in the atmospheric air is electrically charged and collected, a volume of atmospheric air fed into the container can be easily grasped by the flow rate and the drive time of the pump, and further almost all suspended particulate matter in the atmospheric air fed into the container can be collected. Accordingly, a quantity of suspended particulate matter existing in a constant volume of atmospheric air for every particle size can be simply found.